Does Cycles f-stop effect the brightness of the render
No the F stop has no effect in the overall brightness of a scene like it does in real life. It is meant to affect focus and the depth of field primarily.

In real life a opening the iris by one f stop would mean that twice the light would enter the camera making things brighter. 2 stops would be 4 times the amount of light, and so on in a square progression.

That does not happen in blender.

Here's an example:

F stop 0.5:


F stop 22:



Notice how the figure in the middle remains with no brightness change.
In the real world the first image would have been much brighter by several magnitudes.

Here's a simulated illustration of what you would get if by every stop the intensity of the light would change:


To control the overall exposure of a scene in cycles you have several options:

Using the Film->Eposure control.





Depending on what you are after you can also use Nodes on the compositor (Hue/saturation/Value, brightness/contrast/ gamma, or even tone mapping)

Or by messing with the color management's exposure control: